This application claims the priority of German application 199 07 851.3, filed in Germany on Feb. 24, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for shutting down and switching on cylinders of a motor vehicle internal-combustion engine in which the injection valves are controlled by an engine timing system as a function of vehicle operating conditions. The shutting-down of the cylinders is carried out sequentially according to a defined program. The vehicle brakes can be acted upon in a power-operated manner.
Because of stricter emissions laws and the goal of reducing consumption, the principle of shutting down cylinders of an engine is used more and more frequently.
In general, there are two different methods of shutting down cylinders. On the one hand, there is the mechanical, or better, electromechanical shut-down. This operates to shut down cylinders by shutting down the charge cycle valves and switching off the injection valves. This method offers the largest potential with respect to saving fuel. The advantages of the electromechanical shutting-down of cylinders are: (1) low charge cycle losses, (2) low friction losses by the partial shutting-down of the valve gear, (3) low throttling losses, and (4) a better charging of the active cylinders. The disadvantages are: (1) the necessity of a mechanical change at the cylinder head and at the valve gear, (2) the use of an additional electronic control, and (3) the susceptibility of mechanical components, as well as the additional costs and the additional weight of such mechanical components.
A lower-cost implementation for shutting-down of the cylinders involves a purely electronic shutting-down, specifically the shutting-down of the injection valves. Although, in contrast to the mechanical and/or electromechanical shut-down, this method results in a lower reduction of fuel consumption, it is significantly simpler with respect to the expenditures. This is because no mechanical changes must be carried out at the cylinder head and at the valve gear. Only modifications on an already existing control unit are required.
The present shut-down method is one such purely electronic method of shutting-down the cylinders. Concerning the related field of the invention, reference is made to German Patent document DE 196 19 320 A1, German Patent document DE 44 27 203 C1, European Patent document EP 0 647 775 A2 and German Patent document DE 33 13 038 C2.
German Patent document DE 196 19 320 A1 describes a system for shutting down a fixedly defined number of cylinders by stopping the fuel supply. Here, the number of the shut-down cylinders can be changed. For setting the driving comfort, at least one of the ignition angle, the volumetric efficiency and the injection are adjusted. A decision concerning the shut-down takes place as the result of the engine load and the rotational engine speed.
German Patent Document DE 44 27 203 C2 also describes a method of disconnecting individual cylinders by stopping the fuel supply. As a function of a momentary engine load, the process decides whether or not the cylinder following in the ignition order is fired. However, the object of this method is not the saving of fuel, but rather a wheel slip control permitted by this method.
German Patent document DE 33 13 038 C2 describes a method of shutting-down cylinder groups by shutting down the fuel supply and by switching from one cylinder group to another cylinder group in order to avoid a cooling of the shut-down cylinders in view of possible disadvantages in the pollutant emissions. In this case, control takes place by way of the ignition angle and the volumetric efficiency, as well as the air quantity. In addition, the change-over operation is carried out during a certain number of engine revolutions so that, if possible, the driver does not notice any change-over jolt.
A method of the above-mentioned type is described in European Patent document EP 0 647 775 A2, in which a sequential shutting-down of the injection according to a defined program is described for controlling the coolant temperature when idling. A sequential shut-down takes place only if the coolant temperature is higher than 80.degree. C. and/or the air temperature in the intake air collector is higher than 20.degree. C. The alternating shutdown is eliminated only at approximately 5 km/h and is effective only when idling. By means of this method, it is possible to prevent the cylinders from cooling and falling below a defined limit temperature in the case of the catalyst.
The object of the present invention is to further develop a method of the above-mentioned type, in which a change-over jolt is avoided when cylinders are switched on. This object is achieved by a method for shutting down and switching on cylinders in a motor vehicle internal-combustion engine in which the injection valves are controlled by an engine timing system as a function of vehicle operating conditions. The shutting-down of the cylinders is carried out sequentially according to a defined program. The vehicle brakes can be acted upon in a power-operated manner. When cylinders are switched-on, the running-up of the engine takes place in steps according to another defined program, and a braking intervention is carried out for avoiding the change-over jolt.
Accordingly, in the case of a method according to the invention, the engine run-up is carried out in steps according to a defined program. In addition, a braking intervention takes place by power-assisted brakes for avoiding the change-over jolt.
Because of the corresponding control of the injection valves, the purely electronic cylinder shut-down does not offer the maximally possible saving of fuel. However, it can be carried out by means of minimal technical expenditures. The advantages of this method are low throttling losses, a better charging of the active cylinders, and an implementation at reasonable cost. By means of the above-mentioned method, an interconnected overall concept is introduced for the comfort-oriented shutting-down and switching-on of cylinders. In particular, if possible, the driver should not notice the reduction of the operated cylinder or the switching-on of the shut-down cylinders.
Additional measures for avoiding a change-over jolt may include an ignition angle adjustment and a throttle valve adjustment, which are preferably caused by the engine timing system.
A so-called 5/8 shut-down was found to be a suitable shutting-down method in the case of eight-cylinder engines, in which case, in a selective manner, 5 of the 8 cylinders are not operated during two crankshaft revolutions. Nevertheless, over a defined crankshaft angle range, all cylinders are ignited exactly once. More precisely, ignition no longer takes place at every 90.degree. crankshaft angle, but rather at every 270.degree.. The ignition interval as well as the firing of the cylinders to be ignited is correspondingly changed (compare FIG. 3a with FIG. 3b). FIG. 3a shows a normal firing of the cylinder during operation of all cylinders in an 8-cylinder engine.
In the case of an engine with a different number of cylinders, a different number of cylinders can naturally be shut down per two crankshaft revolutions.
In a simple embodiment of the invention, cylinder shut-down takes place only when the vehicle is standing and idling. Although this does not result in a optimal saving of fuel, the momentary jolt problems are reduced.
In another preferred embodiment, cylinder shut-down takes place only when a defined coolant temperature threshold is exceeded. Furthermore, it can be recorded in a defined program that all cylinders of an engine are operated again at least starting from a defined vehicle speed limit.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.